In automated blood-analyzing instruments, such as hematology instruments and fluorescent flow cytometers, it is necessary to mix precisely measured minute volumes, typically of the order of between 5 and 50 microliters, of patient blood with diluents and/or reagents in order to prepare blood samples for analysis. To do this, it is necessary to isolate small volumes of a blood sample aspirated from a vial of blood and to position these isolated volumes so that they may be readily dispensed into different mixing chambers. There are a number of liquid-metering shear valves that are capable of providing these functions; see, e.g., the blood-sampling shear valve assemblies disclosed in the respective disclosures of U.S. Patents to: Coulter et al, U.S. Pat. No. 3,567,389; Cabrera, U.S. Pat. Nos. 4,445,391 and 4,507,977; Cabrera et al., U.S. Pat. Nos. 4,896,546 and 4,702,889; Pellegrino, U.S. Pat. No. 4,822,569; Proni et al., U.S. Pat. No. 4,957,008; Del Valle et al., U.S. Pat. Nos. 5,158,751 and 5,255,568; and Hollinger, U.S. Pat. No. 5,542,305.
All of the shear valve assemblies disclosed in the above patents comprise a stacked set of three disc-shaped pads having confronting planar surfaces. These pads, each being made of ceramic material, are arranged on a common axis about which at least the middle pad of the set is rotatably mounted. The middle pad defines a cylindrically-shaped “segmenting passageway” that extends axially through the entire width of the pad, i.e., between its opposing planar and parallel surfaces. Thus, the volume of this segmenting passageway is determined by the product of its diameter and the thickness of the middle pad. In use, this passageway is used to isolate one of at least two of the required prescribed volumes of blood needed for analysis in a conventional hematology instrument. A second, and somewhat larger, prescribed volume of blood needed for analysis is provided by a hollow external loop that is fluidly connected in series with the segmenting passageway. Thus, a blood sample entering the valve assembly through a port formed in one of the end pads will fill both the segmenting passageway and the hollow external loop, one after the other. As the middle pad is rotated relative to the end pads from a “blood-loading” position, in which blood can enter and fill the two prescribed volumes, and towards a “blood-dispensing” position, in which the two prescribed volumes can be dispensed, the blood flow entering the assembly is sheared off, and the two prescribed blood volumes contained respectively in the segmenting passageway and the external hollow loop are isolated from each other, as well as from the rest of the blood within the valve assembly. Continued rotation of the middle pad towards its blood-dispensing position operates to align the segmenting passageway and the external hollow loop with different ports formed in the respective end pads, thereby enabling the isolated blood volumes to be dispensed to a mixing station or elsewhere. When so aligned, a diluent or reagent that is to be ultimately mixed with the isolated blood sample is used to drive the sample from isolation, such diluent or reagent entering the assembly under pressure through a port in one of the end pads, engaging the isolated sample, and expelling the sample out of the assembly through a port formed in the opposite end pad.
From the above description, it will be appreciated that the shear valve assemblies of the type disclosed in the above patents require at least three valve pads in order to extract or dispense the blood contained in the segmenting passageway. This requirement, of course, adds significant cost and complexity to the valve assembly and can adversely affect its reliability.